Method and apparatus for detecting decrease in tire air-pressure

A method for detecting decrease in tire air-pressure including the steps of: detecting rotational information of the respective tires; storing rotational information; detecting whether the vehicle is running on a flat road or not; detecting whether the vehicle is accelerating or decelerating; comparing rotational information of both front wheels with rotational information of both rear wheels when it has been determined that the vehicle is running on a flat road and that the vehicle is neither accelerating nor decelerating; and judging simultaneous decompression of both front wheel tires or simultaneous decompression of both rear wheel tires on the basis of a relationship between a value obtained through comparison of the rotational information of both front wheels and both rear wheels and a specified threshold. It is possible to judge simultaneous decompression of both front wheel tires or both rear wheel tires when employing the present invention.